1. Field of the Invention
The present invention relates to a method and apparatus for allowing a target cell to most efficiently perform scheduling on packet data service when a serving cell for the packet data service changes from a source cell to a target cell in a mobile communication system supporting packet data service. In addition, the present invention also relates to a fast cell selection method and apparatus for High Speed Downlink Packet Access (HSDPA) used in a Wideband Code Division Multiple Access (WCDMA) system. More particularly, the present invention relates to a method and apparatus for allowing a target cell to rapidly perform HSDPA scheduling when an HSDPA serving cell changes from a source cell to a target cell.
2. Description of the Related Art
A mobile communication system has been developed from an early communication system that provides voice service into a high-speed, high-quality wireless data packet communication system that provides data service and multimedia service. The 3rd Generation (3G) mobile communication system, which is currently divided into 3rd Generation Partnership Project (3GPP) and 3rd Generation Partnership Project 2 (3GPP2), is now under standardization for high-speed, high-quality wireless data packet service. For example, standardization for HSDPA is being conducted in 3GPP, and under standardization for 1xEV-DV is being carried out in 3GPP2. Such standardization can be considered as an attempt to find a solution for high-speed, high-quality wireless data packet transmission service at 2 Mbps or higher in the 3G mobile communication system, and the 4th Generation (4G) mobile communication system aims at providing the high-speed, high-quality multimedia service.
Generally, the HSDPA scheme refers to a scheme for transmitting control information and packet data through a High Speed Dedicated Shared Channel (HS-DSCH), which is a downlink channel for supporting high-speed packet data transmission, in an asynchronous Universal Mobile Terrestrial System (UMTS) mobile communication system.
HSDPA needs an advanced technology which is capable of increasing adaptability to the channel variation in addition to the conventional technology provided in the existing mobile communication system.
Therefore, Adaptive Modulation and Coding (AMC) and Hybrid Automatic Re-transmission Request (HARQ) for supporting high-speed packet transmission have been proposed in HSDPA.
FIG. 1 is a diagram illustrating a process of performing conventional HSDPA serving cell change which uses a Radio Network System Application Part (RNSAP) protocol and a Node B Application Part (NBAP) protocol of WCDMA.
In step 110, a User Equipment (UE) 101 is receiving HSDPA data from a source Node B 102. In step 115, the UE 101 notifies a Serving Radio Network Controller (SRNC) 104 of the fact that a new cell should be added to an active set as a result of monitoring a pilot channel and the like (such an event is called “EVENT 1A” in WCDMA), using an RRC MEASUREMENT REPORT procedure. If the active set is changed, the SRNC 104 sends to the UE 101 a command to change the active set in step 120. Upon receipt of the command, the UE 101 updates the active set and adds the resulting radio link (RL) in step 125, and then proceeds to step 130 where it notifies the SRNC 104 that active set update is completed.
In step 135, the UE 101 notifies the SRNC 104 of the fact that the best cell among the cells included in the active set is changed as a result of monitoring a pilot channel and the like (such an event is called “EVENT 1D” in WCDMA), using an RRC MEASUREMENT REPORT procedure.
Upon recognizing the change in the best cell, the SRNC 104 delivers the information required by a Node B to schedule a UE, such as buffer information, to a target Node B 103 and receives information on the HSDPA resource allocated from the target Node B 103 in step 140, in order to allow HSDPA service to be performed in a new best cell. Step 145 corresponds to a step of instructing the old cell to perform no more HSDPA scheduling. Step 140 and step 145 both use a Synchronous Radio Link Reconfiguration procedure.
In step 150, the SRNC 104 sends to the UE 101 a RADIO BEARER (RB) RECONFIGURATION message with HSDPA configuration information in the new cell. Upon receipt of the RADIO BEARER RECONFIGURATION message from the SRNC 104, the UE 101 delivers an RB RECONFIGURATION COMPLETE message in response thereto to notify the SRNC 104 of correct receipt of the new HSDPA configuration information. Upon receipt of the RB RECONFIGURATION message, the UE 101 monitors High Speed Shared Control Channels (HS-SCCHs) transmitted from the new cell beginning at the arrival of a Connection Frame Number (CFN) included in the message in step 155, to determine whether there is any HSDPA data transmitted thereto.
Because the SRNC 140 used the Synchronous Radio Link Reconfiguration procedure in step 140 and step 145, it sends RL RECONFIGURATION COMMIT messages to the source Node B 102 and the target Node B 103 in step 160, so as to stop HSDPA scheduling in the source cell 102, or the old serving cell, and to start HSDPA scheduling in the target cell 103, or the new serving cell. From now on, the SRNC 140 changes its HSDPA data path from the source Node B 102 to the target Node B 103. Thereafter, the target Node B 103 starts scheduling on the UE 101 in step 165 and step 170.
It can be noted from the foregoing process that there is a considerable time difference between the time where the best cell was changed in step 135 and the time where HSDPA data is actually transmitted from the new best cell 103 in step 165. This problem may be relived to some extent with the use of a method proposed in the present invention in which HSDPA configuration is previously prepared in the target Node B. In addition, it is possible to facilitate fast HSDPA serving cell change by improving the Synchronous Radio Link Reconfiguration procedure performed in the foregoing process.
In the foregoing conventional Synchronous Radio Link Reconfiguration procedure, as the Synchronous Radio Link Reconfiguration procedure commonly defined in RNSAP and NBAP, an SRNC (or Controlling RNC (CRNC)) transmits new radio link configuration information to a Drift RNC (DRNC) (or Node B) along with an RL RECONFIGURATION PREPARE message. Upon receipt of the RL RECONFIGURATION PREPARE message, the DRNC (or Node B) transmits an RL RECONFIGURATION READY message in response thereto to the SRNC (or CRNC) and at the same time, stores the radio link configuration information. Thereafter, upon receipt of the RL RECONFIGURATION COMMIT message for requesting commitment of radio link configuration, the DRNC commits the radio link configuration according to a connection frame number (CFN) included in the received message if the current CFN is equal to the CFN included in the received message.
Currently, because the RL RECONFIGURATION COMMIT message should necessarily include the CFN as shown in Table 1 below, the SRNC delivers this message to the Node B after the best cell is changed, and the new cell performs HSDPA scheduling after a lapse of a predetermined time taking the CFN into account.
TABLE 1IE TypeSeman-AssignedIE/Pres-and Ref-tics De-Critical-Group NameenceRangeerencescriptionCriticalityityMessage TypeMYESignoreTransaction IDM—CFNMYESignoreActive Pattern0FDDYESignoreSequenceonlyInformation